Polarization imaging systems have typically been complex, expensive, and unsuitable for turbid media. Techniques of polarization difference imaging (PDI) systems are conventionally used for capturing a plurality of frames (e.g., images) of a sample. Such techniques are used to determine a spatial difference of the light intensity by comparing one frame of the sample to another.
When looking for a sample with a superficial structure in its single-scattering layer, the light returning from deeper structures can drown out light from a layer of interest. This drowning out occurs because most of the light returning from the sample (for example, 80% of the reflected light in skin) is diffuse. In addition, there is a specular reflection dependent on a refractive index of the sample and the angular extent of the illumination. Such specular reflection makes up roughly 15% of the reflected light. The layer of interest in the superficial single-scattering layer is thus about 4-5% of the reflected light. Removing this background signal allows for highlighting the layer of interest in the superficial structure. Eliminating the background signal is the key principle of the known PDI systems as a contrast enhancement mechanism. In the field of electrical engineering, an electrical circuit manipulating electricity in a similar way that PDI manipulates light is a common-mode rejection amplifier. Such an amplifier is typically utilized to reject noise or background signals (e.g., intensity drift of a light source).
The PDI systems typically include mechanically rotated optical polarizers or tunable liquid crystal polarizers. Techniques of PDI systems usually illuminate a sample with a polarized light and image the sample using at least one polarization sensitive sensor array. In such techniques the light is reflected from a sample surface (i.e., specular reflection) and the light backscattered from the sample surface maintains their wave properties, however, the diffuse light returning from the sample usually loses its polarization properties. This occurs because the light is split between two polarization channels: polarization parallel and polarization perpendicular.
A typical PDI system can be described mathematically as follows:An incident light (PAR)=specular reflection+single-scattering (SS)+½*Diffuse; A cross-polarized light (PER)=½*Diffuse. The PER is orthogonal to the PAR; and PDI=PAR−PER=specular reflection+SS.
Following is a detail description of the two conventional PDI systems discussed in the related art. A PDI system 100 shown with reference to FIG. 1 includes a sample 110 illuminated with a linearly polarized light. This is performed using a light source 120, an illumination optic 122 and a linear polarizer 124. The light (i.e., the specular reflection, the SS, and the diffuse) returning from the sample 110 is split between the two polarization channels by a polarizing beam splitter (PBS) 130. The split light is collected by a plurality of polarization sensitive sensor arrays (SAs) 140-1 and 140-2. Each sensitive SA 140-1, 140-2 is configured to capture at least one frame of the sample 110 respective of the two polarization channels showing in FIG. 1.
The linear polarizer 124 is located with respect to the PBS 130 in such a way that each of the SAs 140 is configured to capture either the PAR or the PER component. The illumination optic 122 is located between the light source 120 and the linear polarizer 124 in a way that enables a user to adjust the beams of light coming out from the light source 120 towards the sample 110. Similarly, a plurality of detection optic units (e.g., detection optic units 150-1 and 150-2) is located between the PBS 130 and each SA 140 to adjust the beams of light towards each SA 140.
Although the PDI system 100 is used by a variety of professionals, the system 100 holds some problems arising from its static nature. As an example, misalignments may occur between the SAs 140. Correcting these misalignments is inefficient in terms of complexity. In addition, a plurality of SAs 140 are required in order to capture the PAR and the PER by the system 100. This also holds disadvantages in terms of costs and complexity. Further, the system 100 is restricted by the polarization separating element (i.e., the PBS 130). Moreover, the system 100 described herein is unsuitable for the polarization of a turbid media.
Another conventional PDI system 200 is shown in FIG. 2. The system 200 illuminates a sample 110 of polarized light. The system 200 captures the PAR and PER components sequentially, as a function of time. That is, the light is polarized by a tunable polarizer 210 to illuminate the sample 110. The tunable polarizer 210 rotates the polarization by 90° between each frame captured by a SA 240, forming a square wave as a function of time. The light (being a combination of the specular reflection, the SS, and the diffuse) returning from the sample 110 is split between the two polarization channels by a liner polarizer 220.
The SA 240 is configured to capture multiple frames of the sample 110. The liner polarizer 220 is oriented either parallel or perpendicular to the polarization of the tunable polarizer 210 to yield PAR and PER images, respectively. The illumination optic 122 is located between the light source 120 and the tunable polarizer 210 to adjust the beams of light coming out from the light source 120 towards the tunable polarizer 210. Similarly, at least one detection optic 230 is located between the linear polarizer 220 and the SA 240 to adjust the beams of light toward the SA 240. In addition, the system 200 requires a control unit, such as a controller 250, to synchronize between the tunable polarizer 210 and the frame capturing function.
Although the system 200 is used by a variety of professionals, the system 200 holds some problems arising from its non-static nature. As an example, interruptions in the rotation of the polarization can occur, thereby causing the system 200 to be inefficient and unreliable. Thus, the rotation of the polarization is inefficient in terms of complexity. In addition, the system is restricted by the polarization separating element (i.e., the linear polarizer 220). Moreover, the system 200 is expensive and unsuitable for turbid media.
The two conventional PDI systems rely on the separation of the various polarizations either in space, onto separate SAs 140, or in time, on separate frames of the same SA 240, using a polarization separating element. Images or sub-images are combined to make an output image of the PAR and PER.
It would be therefore advantageous to provide a solution that overcomes the deficiencies of conventional PDI systems and techniques for separating the light returning from a superficial layer or a surface of a sample and the light returning from one or more deeper layers.